Introduction: Persistent organic pollutants in the environment including pesticides, polychlorinated biphenyl (PCBs) and polyaromatic hydrocarbons (PAHs) are highly toxic and can cause a variety of health effects like cancer, allergies, hypersensitivity, damage to immune and central nervous systems, reproductive disorders and in extreme cases can lead to the death of humans and animals. They readily adsorb to fatty tissue and leads to bio-magnification in the food chain. Guayanilla Bay was exposed to heavy contamination by these chemicals because it was the site of one of the largest petrochemical industrial complexes in the world. Objectives: Long-term goal is to develop a capability for predicting the fate of toxic chemicals in tropical marine environment of Puerto Rico. Specific objectives are to: (1) analyze water quality variables (temperature, turbidity, inorganic nutrients, pH, dissolved oxygen, and salinity) and sediment quality variables (pore water ammonia and pH, and sediment surface dissolved oxygen); (2) determine the types and quantities of pollutants (PAHs, halogenated organics, and metals); (3) determine the bio-availability of these contaminants based on sediment characteristics (grain size, total organic carbon, and acid volatile sulfides); (4) study abiotic and biotic factors affecting biodegradation of PAHs and (5) develop diffusion and dilution models to predict fate of PAHs in tropical marine environment. Research Plan. The overall research effort will include a comprehensive plan for sampling, analysis, quantification of data, laboratory testing and prediction of the fate of PAHs in a tropical setting. Through a comprehensive sampling scheme, analytical methodologies will be employed to measure all variables required in order to assess the physical, chemical and biological conditions for predicting the fate of PAHs. The analytical techniques will provide a detailed analysis of both water and sediment quality variables from which quantification of contaminants especially PAHs will be made. All quality variables will be measured and analyzed using state of art instrumentation. The investigation will culminate into understanding the fate of PAHs through biodegradation pathways. Biodegradation of PAHs will involve the important steps of isolation, enumeration and identification of bacteria involved in the PAHs degradation by the over-layer technique and by the BIOLOG MT Plate system. Diffusion (including the partitioning of PAHs in the water column) and dilution models will be developed upon quantification of all pertinent data.